This invention relates to improvements in motor vehicles, particularly off-road haulage type vehicles such as large ore dump trucks and the like having very large wheels.
Extremely large off-road vehicles are becoming widely used in many industries, particularly in the mining industry and also in the large construction industry, and the like. The excessive size and weight of these vehicles have created many problems in the construction and operation thereof for both efficiency and safety purposes. The wheels of said vehicles are exceptionally large, usually requiring tires in excess of 10 feet in diameter. The power to such vehicles has generally consisted of an engine, transmission, drive line, differential, and rear axle. Time is of the essence in a typical mining situation. The time, that is, the time a loaded vehicle travels between its loading site to its unloading site, and then its return to the loading site, is critical to efficient use of these large off-road vehicles. Many of the mining operations have steep uphill and downhill grades; and due to the desire for high speed, long haul, and long downhill grades, the usual service brakes of these vehicles have not been adequate to retard the speed. It has been found that during braking operation that a common disc brake system, for example, generates a considerable amount of heat, and the metallic material from which the disc is constructed frequently "seizes" the elements to which it is secured, or fuses thereto, resulting in dangerous situations and expensive down time. Hydrodynamic retarders, i.e. water brakes, have been used on such vehicles to relieve the service brakes of the excessive retarding requirements. Typically, however, the retarder is installed in the drive line of the power transmission system.
In later years, the design of such vehicles has progressed to the use of hydraulic and electrical wheel driven vehicles. As such, an electric or hydraulic motor driving means is located in each rear wheel and through a gear reduction system, the entire mechanical drive between the engine and the wheel was eliminated. By installing an alternator/generator or hydraulic pump on the engine, flexibility between the engine and wheels was obtained by transferring power through electric lines or hydraulic tubing. The electric wheel-driven design eliminated the transmission and drive train normally required. Braking of the vehicle was achieved by switching the electric motor to an electric generator during severe downhill braking requirements. The energy developed by the vehicle descending a steep grade is then converted from mechanical energy to electrical energy by the wheels driving the generator/motor. The generated electrical current is then dissipated as heat in an electrical resistance. The heat generated in the electrical resistance is then transferred to the surrounding air by forced convection, i.e. cooling of the electrical resistance. It has been found that resistance of such an electrical regarding system achieves temperatures of 1200.degree. F. to 1300.degree. F. Since the energy transfer rate is directly in proportion with the temperature differential, the four-fold temperature increase resulted in quadrupling the retarded energy. Although the electrical retarding of vehicles resulted in an advantage of continuous retarding, one disadvantage resulted. DC electric motors used on vehicles have a definite torque-speed relationship, but the same characteristic whether the motor is being used as a power source or as a generator absorbing power or energy. As the speed of the motor increases, the torque developed or absorbed decreases. In order for the torque output as a motor or the torque input as a generator to remain constant, the electrical current must increase with speed. The electrical current must flow through the contact between the brushes and the commutators. As the shaft speed increases, the current must be decreased, to prevent arcing or flashover between the brushes and the commutator. Otherwise, the commutator will be damaged. Due to this mechanical limit of electrical current versus speed, the electric motor-driven type of off-road vehicle has very little retarding torque at high speeds, thus restricting the maximum possible speed of the vehicle. If the operator of such a vehicle exceeds the maximum speed on a grade and uses the backup service brakes to slow the vehicle in order to continue down the grade, should he exceed the maximum speed again, there is no means of stopping the vehicle since the service brakes become hot and will fade or become inoperative. For example, a 300,000 lb. vehicle must not exceed 22.5 miles per hour on a twenty percent (20%) grade or 24.5 miles per hour on an eighteen percent grade.